This invention relates to an apparatus for simulating the cure of a pneumatic tire and calculating an equivalent cure therefor. More specifically, this apparatus can be utilized to monitor an actual tire cure and operate the curing press to open the mold when a desired cure state is reached or can be utilized in a laboratory under artificial curing conditions to predict the outcome of a cure or temperature transfer characteristics of a different rubber compound in a particular tire configuration.
The curing or vulcanization of a pneumatic tire is not an exact science, the result of the process varying widely dependent on the rubber compound, tire configuration and enumerable other variables. However, it is highly desirable for the rubber compounder to be able to predict what will occur during the vulcanization of a compound in terms of temperature and corresponding period of time needed to vulcanize each compound or group of compounds used for a variety of shapes and sizes of tires.
To this end, a standard has been developed known as an "equivalent cure," which normally refers to the period of time needed to cure a tire at the equivalent of a reference temperature, usually 300.degree. F. Stated another way, the rubber compounder issues instructions for each particular tire indicating that it must be cured for a certain time period which has been determined by use of some type of equivalent cure calculations.
Previously, the manner in which the compounder makes this determination has varied; but whatever the method utilized, it has long been an empirical process, the compounder never being certain how the addition or substitution of another polymer will change the vulcanization outcome. The most satisfactory method of testing a new compound has been to implant a thermocouple in a green tire and thereafter cure it in a standard tire vulcanization press, that is, one having a curing medium such as steam injected into a curing bag internally of the tire which is positioned in a heated mold. The thermocouple transmits a voltage to a recording pen which plots a curve representing the temperature of the internal points being monitored as a function of time. Once a satisfactory tire is produced from a test cure, the curve derived therefrom is then analyzed to determine the equivalent cure for that compound in the particular tire configuration. The compounder may find, for example, that the thinner sidewall portion of the tire is adequately cured at an equivalent cure (300.degree. standard) of 22 minutes while the thicker tread portion may need 27 minutes of equivalent cure to vulcanize properly. Thus, to prevent an overcuring of the sidewall portions of the tire, an adjustment is made in the compound and the tests run again until approximately one equivalent cure time for the whole tire is established.
While such a process is sufficiently accurate, it has proved quite time consuming and tedious in practice. Initially, the precise placement of the thermocouple involves tedious hand labor requiring much time in threading the wires through each tire, mold, etc. to the recording pen. Further, once the plot is made, then extensive hand calculations are required to determine the time period of the equivalent cure. While such calculations can be performed by a digital computer, usually the expense of the same is not warranted. Finally, it should be evident that the locations of the thermocouples are the only points being monitored and, short of an infinite number of thermocouples, inconsistencies could exist. Thus, there has long been a need for a device which would automatcially record and display the state of cure and calculate the equivalent cure at any instant. Further, it has not been possible to economically perform the desired tests in the laboratory without utilizing an expensive tire vulcanizing press, nor has it been possible to experiment with different temperatures of the curing medium and other variables to determine what the heat transfer effects would be.
It has been known that the heat transfer characteristics of a substance could be simulated by an electronic resistance-capacitance network. Heat transfer is proportional to the conductivity of the material (K) divided by the product of the density (.rho.) and the specific heat (C.sub.v). The known simulation methods can give real time heat transfer simulation by matching resistors to the conductivity factor; matching capacitors to the specific heat of the particular material; and matching the virtual length of the network to the density of the material. However, each time the compound is changed and therefore the heat transfer characteristics changed, the electrical parameters must be adjusted accordingly. For this reason, simulated networks have not been readily usable by the compounder in conducting the aforementioned tests.
Even when the compounder arrives at what he feels, through tedious study and calculation, will be an adequate equivalent cure for a particular compound, the actual curing time of each individual tire may vary appreciably. For example, once it is determined that the compound used in a particular tire must have an equivalent cure of 25 minutes to adequately vulcanize the thick tread shoulder area (which may take 32 minutes of actual vulcanization time), it may be the case that, due to a slightly different compounding or slightly different press conditions or temperatures, some tires would have a 25 minute equivalent cure after 31 minutes of vulcanization. Until now, no device has been available which would either automatically end the curing cycle or tell the operator to do so.
Further, tire compounding often necessitates predictions of the effect of heat build-up in tires during actual running conditions. The standard manner in which this is now accomplished is to implant thermocouples in the tires and run them at a test site where empirical time-temperature curves may be obtained. Again, such a procedure is most time consuming and expensive to undertake.